This invention relates to the recovery of anhydrous hydrogen fluoride from metallic fluoride salts which contain substantial amounts of phosphate values. Such metallic fluoride salts include fluoride containing material precipitated from a waste stream from a wet-process phosphoric acid facility such as pond water. For example, sediment obtained from the floor in gypsum cooling ponds can contain up to 25% fluoride. Similarly, fluoride containing solids can be precipitated from gypsum cooling pond water or waste water by controlling neutralization with limestone and/or lime. These precipitates are contaminated with phosphate and metallic materials.
Conventional reaction of metallic fluoride solids with concentrated sulfuric acid in a low humidity atmosphere to release volatile fluoride values also causes liberation of volatile fluorophosphates. The evolved fluorophosphates rapidly hydrolyze to yield compounds such as HPO.sub.2 F.sub.2 and H.sub.2 PO.sub.3 F, which ultimately reduce to HF and H.sub.3 PO.sub.4. The production of hydrogen fluoride with phosphate values is unsatisfactory because commercial anhydrous hydrogen fluoride requires a HF to P.sub.2 O.sub.5 ratio of greater than 10,000:1.
Therefore, there is a need for a process for producing a nearly phosphate free anhydrous hydrofluoric acid from phosphate-contaminated metallic fluoride salts contained in material such as solids precipitated from cooling pond waters and fluoride containing sludges resulting from phosphate production operations.